Large, multiple engine jet aircraft typically have a fuel system which includes multiple fuel tanks in order to provide long-haul capability while maintaining a proper distribution of weight. In the case of a Boeing 767-300, the aircraft has a primary fuel tank or “wing tank” holding up to 18,500 kgs of fuel in each of the aircraft's wings, and a single auxiliary fuel tank or “center wing tank” holding up to 36,500 kgs of fuel in the aircraft's center wing box. The fuel tanks are fluidly connected to the aircraft's twin engines by a fuel manifold, with the wing tanks and the center wing tank being fluidly connected to the fuel manifold through a cross-feed manifold portion running through the center wing box. This cross-feed manifold portion is divided into two halves by at least one cross-feed valve, with the center wing tank being fluidly connected to both halves of the cross-feed manifold portion in order to create two operationally separate, yet selectively interconnectable, multiple tank subsystems for distributing fuel to the engines.
Fuel is pumped from the primary and auxiliary fuel tanks into the fuel manifold by a plurality of fuel pumps. In the case of a Boeing 767-300, each wing tank includes two low pressure boost fuel pumps, both of which are fluidly connected the same half of the cross-feed manifold portion for sake of redundancy, while the center wing tank includes two high pressure fuel pumps, each of which is fluidly connected to a different half of the cross-feed manifold portion in order to permit independent auxiliary fueling of the aircraft engines. In most aircraft fuel systems, and under ordinary conditions, the auxiliary tank is operated in an override mode, meaning that the low pressure boost fuel pumps in a primary tank remain active regardless of whether fuel is to be drawn from the primary tank or the auxiliary tank. When fuel is to be drawn from the auxiliary tank, a high pressure fuel pump or “override fuel pump” is activated to deliver fuel to the fuel manifold at a greater pressure than the low pressure boost fuel pumps can generate, overriding the flow of fuel from the boost fuel pumps into the fuel manifold and, in effect, de-selecting the primary tank. On the other hand, when fuel is to be drawn from the primary tank, then the override fuel pump is deactivated, and the flow of fuel from the boost fuel pumps into the fuel manifold resumes. In short, in an override-based aircraft fuel system, the system will include both low pressure boost fuel pumps and high pressure override fuel pumps, with the boost fuel pumps remaining active in order to prevent an interruption in fuel flow which could result in an engine flameout, and fuel manifold pressure controlling whether fuel will be drawn from a particular fuel tank.
In some aircraft, factors such as the specific performance capabilities of the installed boost and override fuel pumps, the backpressure profile of the fuel manifold, and the suction pressure generated by an aircraft engine's own fuel pumps can diminish the fuel pressure proximate the boost fuel pumps and cause an “incomplete override” of one or more of the pumps, allowing fuel to be unintentionally drawn from a primary fuel tank, e.g., a wing tank, rather than an auxiliary fuel tank, e.g., a center wing tank. Such a situation may occur, for example, if an engine is operating near maximum power during the aircraft's initial climb and one of the installed boost fuel pumps is capable of pumping at greater-than-average pressures due to pump-to-pump manufacturing variations. Because the amount of fuel drawn from various fuel tanks will vary in an incomplete override scenario depending upon the individual performance capabilities of the fuel pumps, fuel may be drawn unevenly from the aircraft's fuel tanks, adversely affecting the aircraft's distribution of weight, in addition to contravening flight practices which require that auxiliary fuel supplies be essentially exhausted before switching over to primary fuel supplies. This unplanned and, likely, uneven fuel draw may require frequent adjustments to aircraft trim and repeated rebalancing of the aircraft's fuel load, increasing both pilot workload and opportunities for the risks associated with in-flight fuel transfers to become manifest.
The applicant has developed a system and method for supplementing fuel feed pressure and flow without requiring the replacement of existing fuel pumps in existing aircraft, the replacement of existing fuel pump designs in existing aircraft designs, and the like. The disclosed system and method enable the automatic control of a multi-tank, multi-pump, override-based aircraft fuel system using pre-existing fuel pumps or fuel pump designs, in combination with novel modifications to the aircraft's engine monitoring circuitry and fuel system control circuitry, in order to assist the override fuel pump. The system and method thus mitigate a risk of incomplete override with substantially reduced engineering cost and, potentially, substantially reduced maintenance costs and out-of-service delays.